There's been a lot of discussion and analysis over this this thread about this: https://forums.galciv3.com/466422
Admittedly some of it is by Your Truly and I don't want to repeat it all. But the upshot is, things look fine on maps to, oh let's say Large. But after that, it gets hairy. The actual dividing line might be up to personal preference, but I don't think planet generation is porportional as the map sizes increase.
The next thing I might do over in that thread is compare the range of usuable planets generated to tiles and see what the ratios are. Unless someone does that for me.